Hit the Saddle(1937)

Unable to legally capture and sell a herd of protected wild horses, corrupt rancher Rance Macgowan uses his trained killer horse, Volcano, to substitute for the real leader of the herd and cause havoc and death among the ranches. With the government about to drop the restrictions on rounding up the herd, the Three Mesquiteers find themselves in the middle of the controversy after their friend, Sheriff Miller is killed by Volcano.